Bone screws generally comprise an implantable screw capable of internal fixation or anchoring, and can be used to surgically correct many types of injuries or deformities including scoliosis or kyphosis. Bone screws can be used to repair a fracture, or to secure rods, plates or nails, and can be permanent, semi-permanent or bio-degradable. For example, a bone screw may be left in place after a patient heals, or the bone screw may be surgically removed.
Different types of implants (e.g., screws, pins, rods) are sometimes used in spinal surgery to help fix the spine into a more normal position and stabilize a specific area of the spinal column. For example, a cortical screw is a type of bone screw designed to screw into bony prominences of the posterior vertebra or other bone where dense cortical bone is predominantly present, whereas a cancellous bone screw, which has a different thread pattern, is sometimes used during surgery to secure the anterior vertebra or other bone where porous cancellous bone is present in greater quantity. Functioning as firm anchor points, two or more bone screws can be interconnected using locking rods, allowing a spinal segment to be fixated for stabilization or fusion.
Screws for internal fixation can be surgically inserted either open or percutaneously. If they are inserted open, the skin, muscle, and connective tissue are split and retracted surgically, providing open exposure to the underlying bone. Screws and other hardware (e.g., to interconnect the screws) are then implanted, after which the muscle and skin are re-approximated and surgically closed (using sutures or other method).
The most common method for percutaneous insertion of screws for internal fixation is the use of cannulated screws with surgical guide wires. The guide wire, which has a sharp tip that may also be threaded, is mounted to the end of a drill and surgically inserted under fluoroscopic guidance or image guidance until it penetrates through skin and muscle and anchors into bone. Small diameter (1.5 mm or less) guide wires are typically used so that they can be disengaged and repositioned without substantially damaging bone if they do not strike the bone in the desired target. Tools such as dilators, drills, and taps that are cannulated are then positioned over the anchored guide wire and forced down into contact with the bone. These tools are used to prepare the hole to accept the screw. A cannulated screw is then positioned over the guide wire and a cannulated screwdriver is used to insert the screw into bone while the guide wire stays in position. After screw insertion, the guide wire is removed and the puncture required to place the guide wire and screw is sealed.
Pins, screws, and rods can also be used for external fixation. In this case, a portion of the screws and pins reside outside of the body, but pass through the skin and muscle to connect to bone, usually at some distance from the injury or surgical region.
Whether used for internal or external fixation, percutaneous insertion and placement of a bone screw can result in tissue injury. For example, during the insertion procedure, the tap passed over the guide wire or the screw threads may become entangled with connective tissues between the entry point and a bone fixation region which may lead to trauma and/or infection. Moreover, the percutaneous placement of screws can result in the entanglement, entrapment, laceration or compression of one or more nerve fibers that may lead to severe pain, nerve palsy and/or paralysis.
In some cases, using a dilator tube through which the screw is inserted can mitigate this problem. A tube (or sequence of successively larger tubes) is inserted through muscle and connective tissue to form a channel to a position on a bone. A screw can then be passed through the tube and driven into the bone without contacting soft tissues. However, this method can be tedious and it can be difficult to introduce the tube, which must have an inside diameter adequate to pass the screw and screw head, and a substantial wall thickness for tube strength.